1. Field of the Invention
The present invention relates to a process for preparing acrylamide by bringing acrylonitrile into contact with water in the presence of a copper-based catalyst to hydrate acrylonitrile. More specifically, it relates to a process for preparing high-quality acrylamide which permits the manufacture of a polymer having a sufficiently high molecular weight and good water solubility.
2. Description of the Related Art
Acrylamide has been heretofore used in the form of an acrylamide polymer in papermaking chemicals, flocculants, oil recovery agents and the like, and in addition, it has many uses as a comonomer for various kinds of polymers. In the old days, acrylamide for these uses was manufactured by the so-called sulfuric acid method, but in recent years, a contact hydration method which comprises carrying out a reaction in the presence of a copper-based catalyst has been developed. Nowadays, this catalytic hydration method has been industrially practiced instead of the sulfuric acid method.
Among the above-mentioned uses of acrylamide, particularly the flocculants here also recently applied to the treatment of waste water, and in consequence, substantial efforts have been made to improve the quality and performance of acrylamide. Above all, with regard to the acrylamide polymer which can be used as the flocculants, there is a remarkable tendency that its molecular weight which is considered to have a direct influence on the performance is increased, and in recent years, an acrylamide polymer having a high molecular weight of about 10,000,000 or more, particularly about 15,000,000 is required. This molecular weight is much higher as compared with a molecular weight of usually 1,000,000 or less which is required for the acrylamide polymer for other uses or another polymer. In addition, since the obtained acrylamide polymer is usually dissolved in water when used as the flocculants, it is necessary that the acrylamide polymer is promptly dissolved without leaving insolubles. Furthermore, in view of an acrylamide monomer being poisonous, it is required that the unreacted monomer in the polymer be minute, for example, 0.2% by weight or less.
These requirements are incompatible with the increase of the molecular weight, and in order to meet both of them, substantial efforts have now been made. Although this high-molecular acrylamide polymer is only one use of acrylamide, if such acrylamide is not suitable for this use, it is not appplicable for general use. The present invention is concerned with a process for preparing acrylamide which is applicable to many uses inclusive of this use.
The molecular weight referred to in the present invention is a value measured by a test procedure shown in Example 1 which will be hereinafter described. In the case that the polymer obtained in an aqueous medium is dried to form a dry powder having a water content of 20% by weight or less, particularly 10% by weight or less and this dry powder is then used, much attention is paid to water solubility, and the water solubility referred to in the present invention is also mainly employed in this meaning.
The preparation of the acrylamide polymer having the high molecular weight and the sufficient water solubility depends largely upon not only the preparation process of the polymer, but also the quality of the acrylamide. Therefore, in preparing acrylamide from acrylonitrile by the catalytic hydration method, for the purpose of inhibiting the formation of by-products, various methods regarding the improvement of synthetic reaction systems have been reported. In the preparation of acrylamide, it is necessary to sufficiently purify acrylamide, and usually the purification of acrylamide is carried out through an ion exchange resin. However, some kinds of impurities cannot be removed by conventional ion exchange resin purification, and for the sake of the preparation of higher-quality acrylamide, some methods have been suggested.
For example, Japanese Patent Publication No. 12409/1975 (corresponding to U.S. Pat. Nos. 3,911,009 and 3,962,333) have suggested a method in which a copper salt such as copper nitrate or copper acetate is added to a synthetic reaction system to remarkably improve the activity of a copper catalyst, and instead of adding the copper salt, a part of the copper catalyst may be converted into a copper salt, and for this conversion, an inorganic acid or an organic acid is added. In this case, metallic copper is required to be partially oxidized prior to use or simultaneously. Moreover, on pages 23 to 24 of Khim. Technol., 1983 (3), it is described that sulfuric acid or acrylic acid is added to a system using a reduced copper catalyst, thereby improving the reaction rate and selectivity, and this is due to a salt formed by a reaction of the acid with oxides of copper.
Furthermore, according to Japanese Patent Laid-Open No. 57663/1992 corresponding to U.S. Pat. No. 4,820,872 (PCT WO 86/00614), there has been suggested a method in which an oxidizing agent and an oxidized catalyst are dissolved and removed or reducing agent is added, in the presence of a catalyst such as Raney copper; and concretely, a combination of copper nitrate and an organic acid such as acetic acid is described. In consequence, the formation of by-products can be inhibited.
Japanese Patent Laid-open No. 203654/1988 has described that nitric acid or a nitrate and (meth)acrylic acid and/or its salt are added to a synthetic reaction system to prevent the activity of a catalyst and the quality of a monomer from deteriorating. In addition, in preparing acrylamide, a stable operation is possible for a long period of time.
Japanese Patent Publication No. 21220/1986 has described that at least one material selected from the group consisting of ammonia, ureas, aromatic amines, primary and secondary lower alkylamines, and primary and secondary lower alkanolamines is added to a synthetic reaction system to particularly inhibit the secondary formation of impurities which cannot be removed by a conventional ion exchange resin treatment, and an acrylamide polymer which can be prepared from acrylamide obtained under such conditions has a high molecular weight and a sufficient water solubility. Moreover, a strongly acidic cation exchange resin is used in the purification step of acrylamide, whereby these additives can be easily removed.
According to Japanese Patent Laid-open No. 73727/1979, a phenol substituted at its meta-position is added to a synthetic reaction system, whereby the secondary formation of impurities which cannot be removed by a conventional ion exchange resin treatment can be inhibited. Furthermore, a strongly basic anion exchange resin is used in the purification step of acrylamide to easily remove the above-mentioned additive, and an acrylamide polymer which can be prepared from acrylamide obtained under such conditions has a high molecular weight and a sufficient water solubility.
However, Japanese Patent Publication No. 12409/1975 has described a method for preparing acrylamide which comprises bringing acrylonitrile into contact with water in the presence of a copper-based catalyst to hydrate acrylonitrile, and in this case, nitric acid or a nitrate is added. This method is excellent as a means for maintaining and improving a catalyst activity. However, probably due to the accumulation of copper oxidized in a reactor with the elapse of time, it has been admitted that the secondary formation of impurities such as ethylene cyanohydrin increases and the activity of the catalyst deteriorates. The quality of acrylamide can be evaluated on the basis of the water solubility and the molecular weight of the prepared acrylamide polymer, but the quality of acrylamide obtained by this method deteriorates with time.
A method for dissolving copper oxides accumulated in the reactor with an acid or the like disclosed in the above-mentioned Japanese Patent Publication No. 12409/1975, pages 23 to 24 of Khim. Tekhnol., 1983 (3), and Japanese Patent Publication No. 57663/1992 (PCT WO 86/00614) is effective to decrease ethylene cyanohydrin and the like which are impurities caused by the presence of copper oxides. However, the catalyst activity cannot be recovered only by dissolving the copper oxides of the catalyst with acid whose activity has once deteriorated due to the formation of the copper oxides, and in a certain case, the activity further deteriorates. The quality of acrylamide obtained by these methods, i.e., the water solubility and the molecular weight of a polymer obtained by its polymerization are not improved, and when a certain kind of acid is used, it has been admitted that the quality conversely deteriorates.
In the method disclosed in the above-mentioned Japanese Patent Publication No. 21220/1986, at least one material selected from the group consisting of ammonia, ureas, aromatic amines, primary and secondary lower alkylamines, and primary and secondary lower alkanolamines is added to a synthetic reaction system, the catalyst activity noticeably deteriorates and the quality of acrylamide is hardy improved, the reason is probably that the added compound has been absorbed by the catalyst.
In the method of Japanese Patent Application Laid-open No. 73727/1979, a phenol substituted at its meta-position is added to a synthetic reaction system, the deterioration of the catalyst activity does not take place, and when acrylamide prepared by this method is treated with a strongly basic anion exchange resin, the phenol substituted at the meta-position can be removed. The quality of the thus obtained acrylamide is admitted to be higher than that of acrylamide obtained by the method described in Japanese Patent Publication No. 21220/1986 which comprises adding an amine to a synthetic reaction system, but the quality is still insufficient. In addition, the phenol substituted at the meta-position can be removed with a strongly basic anion exchange resin only, and in a removal step, acrylamide itself hydrolyzes to form acrylic acid, so that the exchange capacity of the resin noticeably deteriorates. Moreover, the regeneration of the resin is difficult, and acrylamide is liable to polymerize in the resin layer during the feed of an aqueous acrylamide solution. In consequence, acrylamide obtained by this method is not practical.